1. Technical Field of the Invention
The present invention relates to method and apparatus for optical imaging of retinal function.
2. Description of the Related Art
Currently there are known technologies such as fundus photography, fluorescein angiography, and optical coherent tomography(OCT) as general ophthalmic diagnostic imaging technologies.
The fundus photography is to image a part of the ocular fundus on an optical film or a CCD while illuminating the inside of a patient's eye through the pupil of the eye. Although it is effective to detect hemorrhage of an eye ground, retinal arteriosclerosis, or other abnormalities, it cannot detect the retinal function and the optic disk function.
The fluorescein angiography is to visualize a fluorescent labeling pattern of the microvascular network of the retina by injecting fluorescent agent. Although it is effective to detect defects of a retinal vasculature, it can acquire only indirect information about the retinal function and the optic disk function.
The OCT is to provide an image of the retinal section anatomically. Thus, it does not reflect any functional problem at all.
Therefore, the conventional general ophthalmic diagnostic imaging technologies have been incapable of directly detecting the functional structure of the retina to make use of it for diagnosis or the like.
On the other hand, it is already known that a change is detected in the reflected light intensity of an area where a neural activity arising from stimulation occurs when illuminating a neural tissue such as the cerebral cortex with a light having a certain wavelength. The change is referred to as “intrinsic signal.” The “intrinsic signal” indicates an increase of the deoxyhemoglobin level of the blood arising from the neural activity, an increase in a blood flow or a blood volume, or the change in absorption of light caused by a structural change in the neural tissue.
Furthermore, there is known “an optical imaging method” of continuously recording a reflected light intensity by illuminating the neural tissue with a light having a certain wavelength as means for detecting the intrinsic signal. This optical imaging method facilitates the extraction of a two-dimensional functional structure, for example, referred to as an ocular dominance column of monkey primary visual cortex, by mapping the local activities of the neurons.
Moreover, Patent Document 1 discloses a technology for optical imaging the functional structure of the retina by applying the optical imaging method to the retina.
“The system and method for noninvasively imaging the retinal function” in the Patent Document 1 relates to a system for imaging the change of reflectance of the retina and the intrinsic change caused by the retinal function. As shown in FIG. 1, the system includes an imaging illuminating unit 52 for illuminating retina 51, a retinal stimulation illuminating unit 53 for inducing a functional response, an imaging device 55 for receiving a light from the retina 51 via a retinal imaging optical system 54, image store means 56 for digitalizing and storing the image received from the imaging device 55, and a computer 57 for controlling the system operations and processing the stored image to output a differential function signal corresponding to the retinal function.
[Patent Document 1]
Japanese Unexamined PCT Publication No.2002-521115, “System and Method for Noninvasively Imaging Retinal Function”
The means of the above Patent Document 1 constantly illuminates the retina with a near infrared ray to calculate the change of brightness of a retinal image before and after illuminating the retina with a visible light that activates the neurons of the retina. The change of the intrinsic signal, however, is minute (for example, around 0.1%). Thus, to detect the change, there has been a need to give an examinee anesthesia or the like to fix his or her eyeball strictly during recording.
Therefore, when the above method is applied to a conscious examinee (for example, due to earlier diagnosis) without giving anesthesia or the like, it has been hard to detect a minute difference in the intrinsic signal very reliably because of slight movements of the examinee's eyeball that inevitably occur during recording. Moreover, particularly a patient having a disorder in the retina finds difficulty fixing. In this manner, the problem caused by the movements of an eyeball has been serious.
On the other hand, the human retina has a characteristic that the visual functions concentrate on “the macular area” located in the center where an image comes into a focus. Specifically, the macular area, where cone photoreceptors concentrate most intensively, is the most important area to maintain normal sight. Therefore, it is extremely important to image the function of the macular area and the optic disk in the clinical ophthalmology or the retinal physiology.
According to the means in the Patent Document 1 described above, the discoloration or change in hemoglobin caused by stimulation is relatively small. Therefore, it has a problem that it cannot detect the change of the intrinsic signal that genuinely reflects the photoreceptor function of the macular area.